1. Field of the Invention:
The present invention relates to a transfer sheet which is designed to transfer a printed image having metallic luster to an object. More particularly, it relates to a transfer sheet which is constructed such that the printed image is tranferred together with a laminate of metal layer and printed film layer.
2. Description of the Prior Art:
Heretofore, there has been known a transfer sheet designed to transfer a printed image having metallic luster to an object. It is disclosed in Japanese Patent Publication No. 41915/1980 and U.S. Pat. Nos. 3,494,776, 3,869,336, 3,900,633, 3,975,563, and 3,131,106. The known conventional transfer sheet is made up of a flexible substrate, a release layer having weak adhesion, a metal layer having a printed image, and a pressure sensitive adhesive layer laminated one over another in the order mentioned. The transfer of the printed image is accomplished by pressing the transfer sheet against an object.
In the transfer sheet above, the printed image is required to be transferred to an object having irregular or curved surfaces. To meet this requirement, the printed image layer should preferably be flexible enough to adhere closely to the surface of any configuration. However, this is practically impossible because the metal layer has to have a certain thickness to ensure the satisfactory transfer of printed image.
Although a very thin metal foil is sufficient to impart metallic luster to a printed image, it is easy to break when peeled off from the substrate for transfer to an object. If it is replaced with a thick one, adhesion to curved surfaces would be unsatisfactory.
It is conjectured that if the adhesion of a printed image to an object is increased so that it is easily peeled off from the substrate, it would be possible to prevent the metal layer from breaking during transfer. Experiments to prove this conjecture indicated that a pressure sensitive adhesive having a high adhesion strength makes it difficult to locate a printed image exactly on a desired position. In the case where a plurality of printed images are on one substrate, an excessively tacky pressure sensitive adhesive causes not only desired printed images but also undesired adjacent printed images to be transferred to an object. Also, such an adhesive makes it difficult to adjust the transfer position by slipping a transfer sheet on an object. In the case where a transfer sheet is entirely coated with such an adhesive, the one on non-image parts would also adhere to an object to impair the commercial value.
The disadvantage of excessive adhesion can be overcome by the use of a pressure sensitive adhesive having a low adhesion strength; but it does not perform the complete transfer of a printed image to an object. The low adhesion strength has to be compensated with the uniform pressing against an object. If the pressing is not uniform, there will be variation in adhesion to an object, and that part of the printed image where adhesion is not complete will stay on the substrate, or the printed image is partly damaged, when the transfer sheet is removed from an object.